Field of the Invention
This invention relates to binaural hearing aids, and more particularly, a system architecture for binaural hearing aids. This architecture enhances binaural hearing for a hearing aid user by digital signal processing the stereo audio signals.
Description of Prior Art
Traditional hearing aids are analog devices which filter and amplify sound. The frequency response of the filter is designed to compensate for the frequency dependent hearing loss of the user as determined by his or her audiogram. More sophisticated analog hearing aids can compress the dynamic range of the sound bringing softer sounds above the threshold of hearing, while maintaining loud sounds at their usual levels so that they do not exceed the threshold of discomfort. This compression of dynamic range may be done separately in different frequency bands.
The fitting of an analog hearing aid involves the audiologist, or hearing aid dispenser, selecting the frequency response of the aid as a function of the user's audiogram. Some newer programmable hearing aids allow the audiologist to provide a number of frequency responses for different listening situations. The user selects the desired frequency response by means of a remote control or button on the hearing aid itself.
The problems most often identified with traditional hearing aids are: poor performance in noisy situations, whistling or feedback, lack of directionality in the sound. The poor performance in noisy situations is due to the fact that analog hearing aids amplify noise and speech equally. This can be particularly bothersome when dynamic range compression is used causing normally soft background noises to become annoyingly loud and bothersome.
Feedback and whistling occur when the gain of the hearing aid is turned up too high. This can also occur when an object such as a telephone receiver is brought in proximity to the ear. Feedback and whistling are particularly problematic for people with moderate to severe hearing impairments, since they require high gain in their hearing aids.
Lack of directionality in the sound makes it difficult for the hearing aid user to select or focus on sounds from a particular source. The ability to identify the direction from which a sound is coming depends on small differences in the time of arrival of a sound at each ear as well as differences in loudness level between the ears. If a person wears a hearing aid in only one ear, then the interaural loudness level balance is upset. In addition, sound phase distortions caused by the hearing aid will upset the perception of different times of arrival between the ears. Even if a person wears an analog hearing aid in both ears, these interaural perceptions become distorted because of non-linear phase response of the analog filters and the general inability to accurately calibrate the two independent analog hearing aids.
Another source of distortions is the human ear canal itself. The ear canal has a frequency response characterized by sharp resonances and nulls with the result that the signal generated by the hearing device which is intended to be presented to the ear drum is, in fact, distorted by these resonances and nulls as it passes through the ear canal. These resonances and nulls change as a function of the degree to which the hearing aid closes the ear canal to air outside the canal and how far the hearing aid is inserted in the ear canal.